1. Field of the Invention
Provided herein is an autofocus device that moves an objective lens so that the objective lens is focused at a measurement surface of a measurement object at all times on the basis of the contrast of an image of the measurement object. For example, the autofocus device can be used in an optical measuring device, such as an image measuring device or a microscope, which measures the dimensions and shape of the measurement object while an image of the measurement object is observed with an observation optical system.
2. Description of Related Art
Hitherto, in an optical measuring device, such as an image measuring device or a microscope, an autofocus device that performs focusing a pattern projected on a surface of a measurement object is used. For example, as shown in FIG. 5, an autofocus device discussed in Japanese Unexamined Patent Application Publication No. 2004-29069 includes a light source 1, a projection lens 2, a pattern projection plate 3 (“Pattern Proj. Plate” in FIG. 5), and an advance/retreat mechanism 4 (“A/R Mechanism” in FIG. 5). The pattern projection plate 3 is inserted between the light source 1 and the projection lens 2, and has a predetermined pattern (such as a triangular pattern) formed thereon. The advance/retreat mechanism 4 causes the pattern projection plate 3 to advance into or retreat from an optical path. The advance/retreat mechanism 4 includes a spring and a solenoid. The spring performs a biasing operation so that the pattern projection plate 3 advances into the optical path along a guide mechanism. The solenoid opposes the biasing force of the spring and causes the pattern projection plate 3 to retreat from the optical path. Reference numeral 5 denotes an objective lens, reference numeral 6 denotes a beam splitter, reference numeral 7 denotes a tube lens, and reference numeral 8 denotes a CCD camera.
Therefore, for example, when a material having low contrast, such as a mirror surface or a glass surface, is measured, if the pattern projection plate 3 advances into the optical path, the pattern on the pattern projection plate 3 is projected onto a measurement surface of a measurement object. Therefore, it is possible to perform focusing from the contrast of the pattern. When it is not necessary to project the pattern, if the solenoid is excited, it is possible to retreat the pattern projection plate from the optical path.
However, in the related autofocus device, when a pattern is projected or is not projected, the pattern projection plate is caused to advance into the optical path or retreat from the optical path due to biasing force of the spring or excitation or disexcitation of the solenoid. Therefore, for example, throughput is reduced, the noise generated when performing a switching operation is loud, and the life of the device is relatively short.